1. Field of the Invention
The present invention relates to a color image reader capable of reading both a color image and a black-and-white image.
2. Description of the Related Art
There are two types of color image readers: a color image reader provided with a single white color light source; and a color image reader provided with three light sources for respectively emitting red (R), green (G), and blue (B) color lights. The color image reader with the single white color light source will be referred to simply as "white light image reader" hereinafter. The color image reader with the three light sources will be referred to simply as "three light image reader" hereinafter.
The white light image reader has three color sensors: a red light sensor for detecting a red light component, a green light sensor for detecting a green light component, and a blue light sensor for detecting a blue light component. The three light sensors are constructed from three CCD sensors, each covered with a corresponding color filter. The three color sensors are assembled into a single CCD device. It is noted that the green light sensor has a higher sensitivity, and the blue light sensor has a lower sensitivity. On the other hand, the three light image reader has a single monochromatic sensor covered with no color filter.
Each type of the image readers can read both color images and black-and-white images. When reading a color image, each image reader is controlled to produce R, G, and B signals indicative of red, green, and blue color components of each picture element of the image. When reading a black-and-white image, on the other hand, each image reader is controlled to produce black-white signals BW indicative of whether each picture element presents black or white. It is noted that each image reader can read a color image as a black-and-white image. That is, the image reader can be controlled to read the color image and to produce black-white signals BW indicative of a black-and-white image for reproducing the color image with only black and white colors.
When reading a black-and-white image with the white light image reader, the image reader is controlled to drive the green light sensor to read out the original because the green light sensor has the higher sensitivity. The image reader reads out the black-and-white image at a speed corresponding to the amount of outputs obtainable by the green light sensor. Because the green light sensor can detect red color as black, the green light sensor can also read a color original image as a black-and-white image.
When desiring to read out a black-and-white image with the three light image reader, on the other hand, the image reader is controlled to turn on all the red, green, and blue light sources. The monochromatic sensor is controlled to read out the image with the light from all the three light sources. Accordingly, the monochromatic sensor can produce outputs with amounts almost three times as large as the amounts of outputs obtained by the white light image reader. The three light image reader can therefore read out the image within a shorter period of time.